Leuco dyes and recording materials using the same

ABSTRACT

A leuco dye having the formula ##STR1## wherein R 1 , R 2 , R 3  and R 4  each represent a lower alkyl group; R 5  represents an α-naphthyl group, β-naphthyl group or ##STR2## wherein R 6  represents --NHCOR 9 , a halogen or an amino group which is unsubstituted or substituted by one or two C 1  -C 4  alkyl groups which are the same or different; and R 7 , R 8  and R 9  each represent hydrogen or a lower alkyl group, provided that when both R 7  and R 8  are hydrogen, R 6  represents --NHCOR 9  or bromine. 
     These dyes absorb in the near-infrared region and yield colored images with a color developer therefor with excellent preservability.

This is a division of application Ser. No. 200,674, filed on May 31,1988, now U.S. Pat. No. 4,925,828.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to leuco dyes used for image formation, and whichdyes are capable of yielding colored images having near infraredabsorbance in the range of 600 to 1,000 nm with high stability.

2. Description of the Background

Recording materials using leuco dyes are conventionally known and usedin practice, for example, as pressure-sensitive recording sheets andthermosensitive recording sheets. According to such use, atriphenylmethane leuco dye, a fluoran leuco dye, a phenothiazine leucodye or an auramine leuco dye may be selected because these dyes havetheir own color tone.

Conventionally, such dyes have been developed, with an emphasis onimproving the color tone, that is, on the absorption in the visiblespectrum. Up to the present time, no dyes have been developed which canabsorb in the near infrared region, i.e., a wave length of 700 to 1,000nm, to a sufficient extent.

As the semiconductor laser has become prevalent, a tendency to readrecorded images such as a bar code by use of the semiconductor laser hasgrown, and a demand for a thermosensitive recording sheet and apressure-sensitive recording sheet which can absorb light in the nearinfrared region has also increased.

A variety of such leuco dyes capable of absorbing light in the nearinfrared region has been proposed recently, for example, monovinyl anddivinyl phthalide compounds described in Japanese Laid-Open PatentApplications 51-121035, 51-121037, 51-121038, 57-167979 and 58-157779,and fluorene compounds in Japanese Laid-Open Patent Applications59-199757 and 60-226871. However, the above leuco dyes have theshortcoming that their absorption intensity in the near infrared regionis insufficient. In addition to this shortcoming, they have theshortcoming that the image formation stability is poor, which causesdiscoloration of colored images, and, moreover, the production cost ishigh.

Another near-infrared absorbing leuco dye is proposed in JapaneseLaid-Open Patent Application 60-231766, which exhibits absorption in thenear infrared region, but has the shortcoming that the preservability ispoor, in particular, its heat resistance is poor.

Therefore, a need continues to exist for a leuco dye which can satisfyall of the above-mentioned requirements.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a leucodye which sufficiently absorbs light in the near infrared region andyields colored images with excellent preservability, and moreparticularly a leuco dye with a sufficient absorption intensity in thenear infrared region ranging from 600 to 1,000 nm.

It is also an object of this invention to provide a leuco dye capable ofyielding colored images which can be read by a light source covering avisible spectrum region through the near infrared region and formingcolored images with high stability when used for thermosensitiverecording sheets and pressure-sensitive recording sheets.

According to the present invention, the above and other objects of thepresent invention can be attained by providing a leuco dye having theformula (I), ##STR3## wherein R¹, R², R³ and R⁴ each represent a loweralkyl group; R⁵ represents an α-naphthyl group, β-naphthyl group or##STR4## wherein R⁶ represents --NHCOR⁹, a halogen or an amino groupwhich is unsubstituted or substituted by one or two alkyl groups having1 to 4 carbon atoms, which are the same or different, and R⁷, R⁸ and R⁹each represent hydrogen or a lower alkyl group, provided that when bothR⁷ and R⁸ are hydrogen, R⁶ represents --NHCOR⁹ or bromine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to leuco dyes used for image formation,and more particularly to leuco dyes having a sufficient absorptionintensity in the near infrared region ranging from 600 to 1,000 nm, andwhich are capable of producing images which can be read under a lightsource covering the visible spectrum to the near infrared region. Thepresent dyes yield colored images having remarkable stability when usedas a thermosensitive recording sheet and a pressure-sensitive recordingsheet.

The leuco dyes having the formula (I) of the present invention are inthe form of white or light yellow fine crystals, which are stable inair. The leuco dyes, when colored, exhibit a sufficient absorptionintensity in the near infrared region ranging from 600 to 1,000 nm, andrapidly induce a coloring reaction when coming into contact at themolecular level with electron acceptors (color developers) such asinorganic acids of acidic terra abla and activated clay, organic acids,phenolic compounds and phenolic resins. The above coloring reactionresults in the formation of a green or blue dyestuff, which shows aremarkable preservability such that the dyestuff is considered to bevaluable as a precursor.

The leuco dyes having the formula (I) of the present invention can beeasily synthesized in accordance with the following reaction scheme:##STR5## wherein X⁻ represents an anion of halogen or perchloric acid.

The reaction can be induced by applying heat to the reaction componentsof (II) and (III) in a mixed solvent of methanol and acetic acid.Preparation of the above compound (II) shown in the reaction scheme isdescribed in "Annalen der Chemie, vol. 623, page 204 (1959)."

According to the present invention, the alkyl group represented by R¹through R⁴ and R⁷ through R⁹ in the above formula (I) represents astraight or branched chain lower alkyl group, preferably an alkyl grouphaving 1 to 4 carbon atoms. It is preferable that R⁶ be --NHCOR⁹.

Specific examples of the compounds represented by formula (I) are:##STR6##

The leuco dyes according to the present invention can be employed as acolor-former for use in the recording materials, such as apressure-sensitive recording material, a thermosensitive recordingmaterial and a thermal transfer type recording material in the samemanner as conventional leuco dyes.

When the leuco dyes of the present invention are employed as apressure-sensitive recording material, the pressure-sensitive recordingmaterial can be prepared by the method disclosed in U.S. Pat. No.2,800,457. The main properties required of the leuco dyes for use in thepressure-sensitive recording material are high oil-solubility, excellentcolor development by a color developer, and good image formationstability. With the above properties taken into consideration,appropriate leuco dyes may be selected for such application.

When the leuco dyes according to the present invention are employed as athermosensitive recording material, the thermosensitive recordingmaterial can be prepared by the method described in Japanese Laid-OpenPatent Application 45-14039. In this case, the leuco dyes are requiredto have the following properties: appropriate thermal-sensitive coloringperformance, high degree of whiteness of the background, and sufficientstability of the leuco dyes and the coloring developer thereof. Theappropriate leuco dyes are selected with these properties taken intoconsideration.

In any case, it is important to make the best use of the fact that thecolored image formed by the leuco dyes according to the presentinvention exhibits an excellent absorption intensity in the nearinfrared region.

When the leuco dyes according to the present invention are used as arecording material, they may be employed in combination with other leucodyes contained in conventional thermosensitive recording materials.Since the color tone caused by color formation in the leuco dyesaccording to the present invention is green or blue, the combinationwith other leuco dyes is effective for the correction of the color toneor the light absorption property.

Specific examples of leuco dyes that can be employed in combination withthe leuco dyes according to the present invention are as follows:

3,3-bis(p-dimethylaminophenyl)-phthalide,

3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide

3,3-bis(p-dimethylaminophenyl)-6-diethylaminophthalide,

3,3-bis(p-dimethylaminophenyl)-6-chlorophthalide,

3,3-bis(p-dibutylaminophenyl)-phthalide,

3-cyclohexylamino-6-chlorofluoran,

3-dimethylamino-5,7-dimethylfluoran,

3-diethylamino-7-chlorofluoran,

3-diethylamino-7-methylfluoran,

3-diethylamino-7,8-benzfluoran,

3-diethylamino-6-methyl-7-chlorofluoran,

3-(N-p-tolyl-N-ethylamino)-6-methyl-7-anilinofluoran,

3-pyrrolidino-6-methyl-7-anilinofluoran,

2-[N-(3'-trifluoromethylphenyl)amino]-6-diethylaminofluoran,

2-[3,6-bis(diethylamino)-9-(o-chloroanilino)xanthylbenzoic acid lactam],

3-diethylamino-6-methyl-7-(m-trichloromethylanilino)fluoran,

3-diethylamino-7-(o-chloroanilino)fluoran,

3-butylamino-7-(o-chloroanilino)fluoran,

3-N-methyl-N-amylamino-6-methyl-7-anilinofluoran,

3-N-methyl-N-cyclohexylamino-6-methyl-7-anilinofluoran,

3-diethylamino-6-methyl-7-anilinofluoran,

3-[N,N-diethylamino)-5-methyl-7-(N,N-dibenzylamino) fluoran,

benzoyl leuco methylene blue,

6'-chloro-8'-methoxy-benzoindolino-piropyran,

6'-bromo-3'-methoxy-benzoindolino-piropyran,

3-(2'-hydroxy-4'-dimethylaminophenyl)-3-(2'-methoxy-5'-chlorophenyl)phthalide,

3-(2'-hydroxy-4'-dimethylaminophenyl)-3-(2'-methoxy-5'-nitrophenyl)phthalide,

3-(2'-hydroxy-4'-diethylaminophenyl)-3-(2'-methoxy-5'-methylphenyl)phthalide,

3-(2'-methoxy-4'-dimethylaminophenyl)-3-(2'-hydroxy-4'-chloro-5'-methylphenyl)phthalide,

3-morpholino-7-(N-propyl-trifluoromethylanilino)fluoran,

3-pyrrolidino-7-trifluoromethylanilinofluoran,

3-diethylamino-5-chloro-7-(N-benzyl-trifluoromethylanilino)fluoran,

3-pyrrolidino-7-(di-p-chlorophenyl)methylaminofluoran,

3-diethylamino-5-chloro-7-(α-phenylethylamino)fluoran,

3-(N-ethyl-p-toluidino)-7-(α-phenylethylamino)fluoran,

3-diethylamino-7-(o-methoxycarbonylphenylamino)fluoran,

3-diethylamino-5-methyl-7-(α-phenylethylamino)fluoran,

3-diethylamino-7-piperidinofluoran,

2-chloro-3-(N-methyltoluidino)-7-(p-n-butylanilino)fluoran,

3-(N-benzyl-N-cyclohexylamino)-5,6-benzo-7-α-naphthyl-amino-4'-bromofluoran,and

3-diethylamino-6-methyl-7-mesidino-4',5'-benzofluoran.

As the color developers for use in combination with the above leuco dyesin the present invention, a variety of electron acceptors or oxidizingagents capable of inducing color formation in the leuco dyes can beemployed in order to obtain a recording material.

Specific examples of such conventional color developers are bentonite,zeolite, acidic terra abla, activated clay, colloidal silica, zincoxide, zinc chloride, zinc bromide, aluminum chloride, salicylic acid,3-tert-butylsalicylic acid, 3,5-di-tert-butylsalicylic acid,di-m-chlorophenyl thiourea, di-m-trifluoromethylphenyl thiourea,diphenylthiourea, salicylanilide, 4,4'-iso-propylidenediphenol,4,4'-isopropylidenebis(2-chlorophenyl),4,4'-isopropylidenebis(2,6-dibromophenol),4,4'-isopropylidenebis-(2,6-dibromophenol),4,4'-isopropylidenebis-(2-methylphenol),4,4'-isopropylidenebis(2,6-dimethyl-phenol),4,4'-isopropylidenebis(2-tert-butylphenol), 4,4'-sec-butylidenediphenol,4,4'-cyclohexylidenebisphenol, 4,4'-cyclohexylidenebis(2methylphenol),4-tert-butylphenol, 4-phenylphenol, 4hydroxydiphenoxide, α-naphthol,β-naphthol, 5-hydroxy dimethyl phthalate,methyl-4-hydroxybenzoate,4-hydroxyacetophenone, novolak-type phenolicresin, 2,2'-thiobis(4,6-dichlorophenol), catechol, resorcinol,hydroquinone, pyrogallol, phloroglucine, phloroglucinocarboxylic acid,4-tert-octylcatechol, 2,2'-methylene-bis-(4-chlorophenol),2,2'-methylenebis(4-methyl-6-tert-butyl-phenol),2,2'-dihydroxy-diphenyl, ethyl p-hydroxybenzoate, propylp-hydroxybenzoate, butyl p-hydroxybenzoate, benzyl p-hydroxybenzoate,p-chlorobenzyl p-hydroxybenzoate, o-chlorobenzyl p-hydroxybenzoate,p-methylbenzyl p-hydroxy-benzoate, n-octyl benzoic acidp-hydroxybenzoate, benzoic acid, zinc salicylate, 1-hydroxy-2-naphthoicacid, 2-hydroxy-6-naphthoic acid, 2-hydroxy-6-zinc naphthoate,4-hydroxydiphenylsulfone,4,2'-diphenol sulfone,4-hydroxy-4'-chlorodiphenyl sulfone, 4-hydroxy-4'-benzyloxy diphenylsulfone, 4-hydroxy-4'-isopropoxy diphenyl sulfone, 4-hydroxy-4'-isobutyloxydiphenyl sulfone, bis(4-hydroxyphenyl)sulfide, 2-hydroxy-p-toluicacid, 3,5-di-tert-zinc butyl salicylate, 3,5-di-tert-tin butylsalicylate, tartaric acid, oxalic acid, maleic acid, citric acid,succinic acid, stearic acid, 4-hydroxyphthalic acid, boric acid,biimidazole, hexaphenyl biimidazole, carbon tetrabromide,methylenebis-(oxyethylenethio)diphenol,ethylenebis-(oxyethylenethio)diphenol,bis(4-hydroxyphenylthioethyl)ether, andm-xylylenebis(4hydroxyphenylthio)ether.

In order to obtain a pressure-sensitive recording material by use of theleuco dyes according to the present invention, the leuco dye isdissolved in alkylnaphthalene or tarphenyl oil with a ratio of 1 to 4%,to which a resin such as gelatin is added to form microcapsules having adiameter of about 5 μm. Such microcapsules are coated onto a supportsuch as a sheet of paper or plastic film by use of a binder, whereby acolor former sheet is prepared. Such microcapsules can be prepared by aconventional method, for instance, by the method described in U.S.patent Ser. No. 2,800,457. A color developer sheet can be prepared bydispersing and dissolving a color developer in water or an organicsolvent by means of an appropriate dispersing medium, adding a binderagent thereto when necessary, and then coating the dispersion on thesupport such as a sheet of paper.

In order to prepare a thermosensitive recording material by use of theleuco dye according to the present invention, the leuco dye and thecolor developer are individually dispersed with addition of awater-soluble resin, and an appropriate binder is added to thedispersion. The thus prepared mixture is coated onto a support such as asheet of paper. It is preferable that the particle size of the leuco dyebe from 0.5 to 5.0 μm and that the weight ratio of the leuco dye to thecolor developer in the mixture be from 1:1 to 1:5. The coloring layermay be formed by coating a coating liquid at a time or two timesseparately. Furthermore, a leuco dye layer and a color developer layermay be separately coated onto the support. An undercoat layer and/or aprotective layer may be provided as known in the preparation ofconventional thermosensitive recording materials.

When a thermal image transfer type recording material is prepared by useof a leuco dye according to the present invention, the leuco dye isdispersed or dissolved in water or a solvent to prepare athermal-transfer type recording layer coating liquid. This coatingliquid is coated onto a heat-resistant support such as a polyester filmto form an image transfer sheet. An image receiving sheet can beprepared by dispersing or dissolving the color developer in water or asolvent to prepare a coating liquid and coating the liquid onto asupport sheet.

In the present invention, a variety of conventional binder agents can beemployed for binding the above-mentioned leuco dyes and color developersto the support for a thermosensitive recording material, and forsupporting leuco dyes in the form of capsules and color developers onthe support for a pressure-sensitive recording material.

Specific examples of such binder agents are as follows: polyvinylalcohol; starch and starch derivatives; cellulose derivatives such asmethoxycellulose, hydroxyethylcellulose, carboxymethylcellulose,methylcellulose and ethylcellulose; water-soluble polymeric materialssuch as sodium polyacrylate, polyvinyl pyrrolidone, acrylamide/acrylicacid ester copolymer, acrylamide/acrylic acid ester/methacrylic acidcopolymer, styrene/maleic anhydride copolymer alkali salt,isobutylene/maleic anhydride copolymer alkali salt, polyacrylamide,sodium alginate, gelatin and casein; and latexes of polyvinyl acetate,polyurethane, styrene/butadiene copolymer, polyacrylic acid, polyacrylicacid ester, vinyl chloride/vinyl acetate copolymer,polybutylmethacrylate, ethylene/vinyl acetate copolymer andstyrene/butadiene/acrylic acid derivative copolymer.

Further in the present invention, auxiliary additive components whichare used in the conventional thermosensitive and pressure-sensitiverecording materials, such as a filler, a surface active agent, athermofusible material, a lubricant and a preventive agent of colorformation owing to application of the pressure, can be employed,together with the above-mentioned leuco dyes and color developers.

Specific examples of a filler for use in the present invention arefinely-divided inorganic powders of calcium carbonate, silica, zincoxide, titanium oxide, aluminum hydroxide, zinc hydroxide, bariumsulfate, clay, talc, surface-treated calcium and surface-treated silica,and finely-divided organic powders of urea - formaldehyde resin,styrene/methacrylic acid copolymer, and polystyrene.

As the lubricant, for example, higher fatty acids, metallic salts,amides, esters thereof, and a variety of waxes such as animal,vegetable, mineral and petroleum can be used.

The recording material containing leuco dyes for use in the presentinvention can be employed in various applications just like aconventional dye. In particular, since the leuco dyes of the presentinvention have excellent absorption intensity in the near infraredregion, such leuco dyes can be utilized in an optical character readingapparatus, and can be employed as a recording material for a label barcoder and a bar code reader.

When the recording material containing leuco dyes for use in the presentinvention are employed as a thermosensitive recording adhesive labelsheet, a thermosensitive color-forming layer containing the above leucodyes and the color developer is formed on the front side of the support,and an adhesive layer is formed on the back side of the support, with adisposable backing sheet attached to the adhesive layer.

The present invention will now be explained in detail with reference tothe following examples which are provided merely for purposes ofillustration, and are not intended to limit the present invention.

EXAMPLE 1 [Synthesis ofbis(p-dimethylaminostyryl)-α-naphthylsulfonylmethane (Compound No. 1)]

2.8 g of bis(p-dimethylaminostyryl)carbonium perchlorate and 1.9 g ofacetate were dissolved in 80 me of methanol. To this solution, 80 me ofa methanol solution containing 4.1 g of sodium α-naphthyl sulfinate wasadded dropwise and refluxed for 1 hour. The resulting precipitate wasseparated from the solution by filtration and recrystallized from amixed solvent of benzene and hexane, whereby Compound No. 1 was obtainedin the form of light yellow crystals having a melting point of 178° to183° C. The yield was 3.1 g.

EXAMPLES 2 AND 3

Example 1 was repeated except that sodium α-naphthyl sulfinate employedin Example 1 was respectively replaced by the sodium sulfinatederivatives No. 2 and No. 3 in Group A in Table 1, whereby Compound No.2 and Compound No. 3 were obtained.

EXAMPLE 4

Example 1 was repeated except that bis(p-dimethylaminostyryl)carboniumperchlorate employed in Example 1 was replaced bybis(p-diethylaminostyryl)carbonium perchlorate, whereby Compound No. 6was obtained.

EXAMPLE 5

Example 1 was repeated except that bis(p-dimethylaminostyryl)carboniumperchlorate employed in Example 1 was replaced by bisp-(N-methyl-N-ethylamino)styryl carbonium perchlorate, whereby CompoundNo. 9 was obtained.

EXAMPLE 6

Example 1 was repeated except that bis(p-dimethylaminostyryl)carboniumperchlorate employed in Example 1 was replaced by bis(4-dimethylamino-2-methylstyryl)carbonium perchlorate, whereby CompoundNo. 10 was obtained.

The color tones and the melting points of five Compounds obtained byExamples 2 through 6 are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                        Ex-                                   Melting                                 am-                    Pro-    Color  Point                                   ples Group A           duct    Tone   (°C.)                            ______________________________________                                              ##STR7##         Com- pound No. 2                                                                      Light yellow                                                                         182˜186                           3                                                                                   ##STR8##         Com- pound No. 3                                                                      Light yellow                                                                         133˜150                           4                                                                                   ##STR9##         Com- pound No. 6                                                                      Slightly pale yellow                                                                 153˜161                           5                                                                                   ##STR10##        Com- pound No. 9                                                                      Slightly pale yellow                                                                 155˜165                           6                                                                                   ##STR11##        Com- pound No. 10                                                                     Slightly pale yellow                                                                 85˜90                             ______________________________________                                    

Embodiments of the leuco dye according to the present invention will nowbe explained with reference to the following examples.

EXAMPLES 1 TO 6, AND COMPARATIVE EXAMPLES 1 TO 6

A dispersion A and a dispersion B were separately prepared by dispersingthe following respective components in a ball mill:

    ______________________________________                                                        Parts by Weight                                               ______________________________________                                        [Dispersion A]                                                                Each leuco dye in Table 2                                                                       10                                                          10% aqueous solution                                                                            10                                                          of polyvinyl alcohol                                                                            30                                                          Water                                                                         [Dispersion B]                                                                Stearic acid amide                                                                              20                                                          Calcium carbonate 20                                                          4,4'-diphenol sulfone                                                                           30                                                          10% aqueous solution                                                                            35                                                          of polyvinyl alcohol                                                          Water             210                                                         ______________________________________                                    

The thus obtained dispersion A and the dispersion B were mixed in aratio by weight of 1 to 1, so that a coating liquid was prepared. Thiscoating liquid was coated into a sheet of wood free paper in such amanner that the deposition of leuco dye component was 0.6 g/m² on a drybasis, and then dried, whereby thermosensitive recording materials No. 1through No. 6 according to the present invention were prepared.

In the same manner as described above, comparative thermosensitiverecording materials Nos. 1 to 6 were prepared by use of the respectiveleuco dyes in Table 2.

The thus prepared thermosensitive recording materials Nos. 1 to 6according to the present invention and the comparative thermosensitiverecording materials Nos. 1 to 6 were subjected to a thermal printingtest by use of a commercially available facsimile equipment (Trademark"Rifax 130" made by Ricoh Company, Ltd.).

The density of the formed images and the background density weremeasured by use of a Macbeth densitometer 914 with a filter W-106.

The absorbance was measured by a reflective spectrophotometer (Trademark"Hitachi 330-type Spectrophotometer" made by Hitachi, Ltd.), and thenthe PCS value of each printed sample was obtained by the followingformula: ##EQU1##

Further, printed samples of the above recording materials were subjectedto a heat resistant test by allowing each printed sample to stand at 60°C. for 24 hours. Thereafter, the density of the formed images and thebackground density were measured in the same manner as mentioned above,and the PCS value of each printed samples, which is defined by the aboveformula, was obtained after the heat resistance test.

The results of the above tests are shown in Table 2. From Table 2, itcan be seen that recording materials prepared using the present leucodyes yield images having a good absorption intensity in thenear-infrared absorption range of from 600 to 1,000 nm with highstability.

                                      TABLE 2                                     __________________________________________________________________________                    Immediately After                                                                             After Heat-resistance                                         Printing Test   Test                                                          Colored                                                                            Back-      Colored                                                                            Back-                                                    Image                                                                              ground                                                                             PCS Value                                                                           Image                                                                              ground                                                                             PCS Value                           No.    Leuco Dye                                                                              Density                                                                            Density                                                                            at 900 nm                                                                           Density                                                                            Density                                                                            at 900 nm                           __________________________________________________________________________    Ex. 1  Compound No. 1                                                                         1.26 0.08 90% or                                                                              1.26 0.10 90% or                                                        more            more                                Ex. 2  Compound No. 2                                                                         1.23 0.09 90% or                                                                              1.22 0.10 90% or                                                        more            more                                Ex. 3  Compound No. 3                                                                         1.20 0.10 90% or                                                                              1.18 0.09 90% or                                                        more            more                                Ex. 4  Compound No. 6                                                                         1.26 0.09 90% or                                                                              1.25 0.09 90% or                                                        more            more                                Ex. 5  Compound No. 9                                                                         1.25 0.08 90% or                                                                              1.22 0.09 90% or                                                        more            more                                Ex. 6  Compound No. 10                                                                        1.24 0.08 90% or                                                                              1.24 0.09 90% or                                                        more            more                                Comparative                                                                          Bis(p-dimethyl-                                                                        1.25 0.09 90% or                                                                              1.23 0.12 70% or                              Ex. 1  aminostyryl)-p-    more            less                                       tolylsulfonyl-                                                                methane                                                                Comparative                                                                          Crystal Violet                                                                         1.29 0.09 10% or                                                                              1.19 0.13 10% or                              Ex. 2  lactone            less            less                                Comparative                                                                          3-diethylamino-                                                                        1.30 0.09 10% or                                                                              1.28 0.12 10% or                              Ex. 3  6-methyl-7-        less            less                                       anilinofluoran                                                         Comparative                                                                          Bis(p-dimethyl-                                                                        1.26 0.12 75% or                                                                              1.20 0.12 70% or                              Ex. 4  aminostyryl)-      less            less                                       benzenesulfonyl-                                                              methane                                                                Comparative                                                                          Bis(p-dimethyl-                                                                        1.23 0.10 75% or                                                                              1.18 0.10 70% or                              Ex. 5  aminostyryl)-      less            less                                       p-chlorobenzene-                                                              sulfonyl-methane                                                       Comparative                                                                          Bis(p-dimethyl-                                                                        1.21 0.11 75% or                                                                              1.20 0.11 70% or                              Ex. 6  aminostyryl)-p-    less            less                                       dimethylamino-                                                                benzenesulfonyl-                                                              methane                                                                __________________________________________________________________________

EXAMPLES 13 TO 16

Example 1 was repeated except that Dispersion B was replaced by thefollowing Dispersion C, whereby recording materials Nos. 7 to 10according to the present invention were prepared:

    ______________________________________                                                         Parts by Weight                                              ______________________________________                                        [Dispersion C]                                                                Stearic acid amide 20                                                         Calcium carbonate  20                                                         Each compound in Group B                                                                         30                                                         in Table 3                                                                    10% aqueous solution                                                                             35                                                         of polyvinyl alcohol                                                          Water              210                                                        ______________________________________                                    

The recording materials No. 7 to No. 10 according to the presentinvention were subjected to the same evaluation tests as in Example 7.The results are shown in Table 3.

As can be seen from the results in Table 3, the recording materialscontaining the present leuco dyes are capable of yielding images havinga good absorption intensity in the near infrared region ranging from 600nm to 1,000 nm, with high stability.

                                      TABLE 3                                     __________________________________________________________________________                  Immediately After                                                                             After Heat-resistance                                         Printing Test   Test                                                          Colored                                                                            Back-      Colored                                                                            Back-                                          Leuco Dyes                                                                              Image                                                                              ground                                                                             PCS Value                                                                           Image                                                                              ground                                                                             PCS Value                             No. in Group B                                                                              Density                                                                            Density                                                                            at 900 nm                                                                           Density                                                                            Density                                                                            at 900 nm                             __________________________________________________________________________    Ex. 7                                                                             4-hydroxy-4'-iso-                                                                       1.24 0.08 90% or                                                                              1.22 0.09 90% or                                    propoxy-diphenyl-   more            more                                      sulfone                                                                   Ex. 8                                                                             4-hydroxy-4'-                                                                           1.24 0.08 90% or                                                                              1.25 0.10 90% or                                    benzyloxydiphenyl-  more            more                                      sulfone                                                                   Ex. 9                                                                             4,4'-thiobis(2-                                                                         1.23 0.08 90% or                                                                              1.22 0.09 90% or                                    tert-butyl-5-       more            more                                      methylphenol)                                                             Ex. 10                                                                            4,4'-cyclo-                                                                             1.24 0.08 90% or                                                                              1.26 0.09 90% or                                    hexylidene-         more            more                                      diphenol                                                                  __________________________________________________________________________

Having now fully described this invention, it will be apparent to one ofordinary skill in the art that many changes and modifications can bemade thereto without departing from the spirit or scope of the inventionas set forth herein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A leuco dye having the formula (I), ##STR12##wherein R¹, R², R³ ad R⁴ each represent a lower alkyl group; R⁵represents an α-naphthyl group, β-naphthyl group or ##STR13## wherein R⁶represents --NHCOR⁹, a halogen or an amino group which is unsubstitutedor substituted by one or two C₁ -C₄ alkyl groups which are the same ordifferent; and R⁷, R⁸ and R⁹ each represent hydrogen or a lower alkylgroup, provided that when both R⁷ and R⁸ are hydrogen, R⁶ represents--NHCOR⁹ or bromine.
 2. The leuco dye as claimed in claim 1, wherein R⁵represents ##STR14## and R⁷ and R⁸ and R⁹ each represent hydrogen or alower alkyl group.
 3. The leuco dye as claimed in claim 1, wherein saidlower alkyl group represented by R⁷ and R⁸ is an alkyl group having 1 to4 carbon atoms.
 4. The leuco dye as claimed in claim 2, wherein saidalkyl group represented by R⁷, R⁸ and R⁹ is an alkyl group having 1 to 4carbon atoms.
 5. The leuco dye as claimed in claim 1, wherein R⁵represents an α-naphthyl group or β-naphthyl group, and R⁷ and R⁸ eachrepresent hydrogen or a lower alkyl group.
 6. A dye-containingcomposition, comprising:a) one or more leuco dyes having the formula(I): ##STR15## wherein R¹, R², R³ and R⁴ each represents a lower alkylgroup; R⁵ represents an α-naphthyl group, β-naphthyl group or ##STR16##wherein R⁶ represents --NHCOR⁹, a halogen or an amino group which isunsubstituted or substituted by one or two C₁ -C₄ alkyl groups which arethe same or different; and R⁷, R⁸ and R⁹ each represent hydrogen or alower alkyl group, provided that when both R⁷ and R⁸ are hydrogen, R⁶represents --NHCOR⁹ or bromine; and b) one or more other leuco dyescapable of correcting the color tone or the light absorbing propertiesof the leuco dye or dyes of the formula (I).
 7. A dye-containingcomposition, comprising:one or more leuco dyes having the formula (I):##STR17## wherein R¹, R², R³ and R⁴ each represents a lower alkyl group;R⁵ represents an α-naphthyl group, β-naphthyl group or ##STR18## whereinR⁶ represents --NHCOR⁹, a halogen or an amino group which isunsubstituted or substituted by one or two C₁ -C₄ alkyl groups which arethe same or different; and R⁷, R⁸ and R⁹ each represent hydrogen or alower alkyl group, provided that when both R⁷ and R⁸ are hydrogen, R⁶represents --NHCOR⁹ or bromine; and b) one or more electronacceptor-color developers capable of rapidly inducing a coloringreaction when in contact with the leuco dyes of the formula (I).
 8. Theleuco dye as claimed in claim 1, which is selected from the groupconsisting of: ##STR19##